The present invention concerns a method for cutting worm screws comprising two adjacent threaded portions having respective threads which are of opposite hand. The invention also relates to a worm screw obtained by such a method.
In our French Patent Ser. No. 2,372,998 and in our Certificate of Addition Application No. 79-15796 thereto, there has already been proposed a gear transmission device comprising a driving shaft and driven shaft fixed in relation to each other, one of the shafts carrying two threads of opposite hand and being connected to the other shaft by two different kinematic gear drive trains, each comprising one of the two above-mentioned threads. Since this device has given every satisfaction as regards its transmission efficiency, it has now been attempted to manufacture economically such shafts carrying two adjacent worm screw threaded portions having their respective threads of opposite hand.
Amongst the various known cutting methods, it has been found to be worthwhile to effect the cutting on a lathe, and since the depth of thread necessary for the worm screw is considerable, provision has been made for the cutting to be effected during several successive passages of the cutting tool. For such a worm screw, the external diameter is fixed, on the one hand, by the depth of the thread necessary for a good meshing with the pinions associated with the worm screw and, on the other hand, by the diameter of the root circle (i.e. the residual part near the axis) of the screw, which diameter is determined by the magnitude of the torques to be transmitted. The length of the threaded portions to be cut on the original cylindrical shaft is governed by the need to have adequate meshing of the worm threads with the threads of pinions cooperating therewith, but since the transmission between the input shaft and the output shaft is effected by two different kinematic gear trains, one of these kinematic gear trains comprises an intermediate pinion of relatively large diameter, (the diameter of this intermediate pinion being all the greater when the interspacing is increased between the two threaded portions of opposite hands along the cylindrical shaft on which the threads are cut.). It is therefore clear that in order to reduce the dimensions of the transmission mechanism using such a worm screw, it is desirable to bring the two threaded portions of the same shaft as close to each other as is at all possible.
On the other hand, it is known that when cutting worm screw threads on a lathe, the worm screw is continuously turning and the cutting tool may effect several successive passes to cut the desired thread. In the case of cutting a deep thread, this cutting operation is effected with a considerable number of passes and, at the beginning of each pass, the tool must be brought radially inwardly towards the axis of the shaft to be cut. In this approaching motion of the cutting tool, it must be prevented from abutting the workpiece with one of its distinct cutting edges. To reduce the risk of contact by the tool, it is possible to increase the rake angle of the tool, but in practice in this field the margin for manoeuvre is extremely limited because the cutting angle of the tool is fixed by (a) the cutting speed, (b) the nature of the material cut and (c) the nature of the tool, and the angle of the cutting edge of the tool should be as large as possible to avoid any excessive fragility of the tool, e.g. embrittlement of the tool. In practice, the rake angle of the tool for cutting a worm screw of the desired type cannot therefore be greatly modified and is always near 6.degree.. To avoid tool contact, the approach of the tool must be controlled, in terms of the rotation of the shaft being cut so that the cutting tool describes in relation to the shaft a trajectory in the shape of a twisted curve and making it possible to reach the zone to be cut without striking on the shaft material. The projection of this twisted curve on a plane perpendicular to the axis of the shaft being cut is a spiral arc and its projection on a plane passing through the axis of the shaft to be cut constitutes a curve segment whose ends define a chord forming an acute angle with the axis of the shaft to be cut. In other words, the motion of approach of the tool is obtained by displacing the tool in the axial direction of the shaft to be cut (and in the direction of the thread) at the same time as it is brought closer to the axis of the shaft to be cut. This motion of approach is all the more delicate the nearer the tool action is to the axis of the shaft to be cut, that is to say that the trajectory of the approach is defined for the last cutting pass but for convenience the approach trajectory of the tool is kept at this same form for all the earlier passes.
As has been indicated above, it is very desirable for a gear mechanism of the type specified in French Pat. No. 2,372,998--to obtain a maximum proximity for the adjacent ends of the two threaded portions of opposite hand. If, after the first such threaded portion has been cut, the cutting of the second portion is commenced by proceeding in such a way that the two adjacent ends of the two threaded portions are substantially in the same transverse plane of the cut shaft, there will be a difficulty by reason of the fact that in its motion of approach for each pass of the cutting of the second threaded portion, the cutting tool encounters the previously cut thread of the first threaded portion, and this entails shock loads on the cutting tool which rapidly lead to its deterioration. To avoid this drawback, one way of course interspace the two nearer ends of the two adjacent threaded portions of the worm screw, but then the size of the resulting gear mechanism using the worm screw rapidly increases as does the cost of the worm screw and that of the complete gear mechanism. Thus one is faced with a problem: if it is desired to reduce the cost by reducing the dimensions, the cutting tool will deteriorate and the production cost will increase whereas it is precisely in order to reduce the said cost that production by lathe has been chosen. If the increase in production costs is avoided by ensuring the long life of the cutting tool of the lathe, the size of the worm screw is increased and hence the material cost of the mechanism as a whole.